Single molecule-single nanoparticle surface enhanced Raman scattering event is analyzed using a quantum mechanical approach, resulting in an analytical expression for the electromagnetic enhancement factor that succinctly elucidates the fundamental aspects of SERS. The nanoparticle is treated as a dielectric spherical cavity, and the resulting increase in the spontaneous emission rate of a molecule adsorbed onto the surface of the nanoparticle is examined. The overall enhancement in Raman scattering is due to both the increased local electromagnetic field and the Purcell effect. The predictions of the present model are in agreement with the simulation results of the classical model.